The Organic Semiconductor Material Carrier Mobility Theoretical Simulation

Due to potential utility in organic optoelectronic devices such as light emitting diodes (LEDs), field effect transistors (FETs), photovoltaic devices (OPV), organic semiconductors are paid much attention by scientific and industry field recent years. The conductive property of molecules is a key parameter to achieve high efficiency in devices. The charge mobility is a main property to evaluate the conduction of molecular materials. For most organic materials, the interaction between the molecules is very weak compared with electron-phonon coupling, thus, the charge is transport by thermal activated hopping way. According to the classic semi-empirical Marcus electron transfer theory, there are two important factors that affect charge transfer rate. One is the charge transfer integral, the other is reorganization energy caused by electron transfer process. Therefore, fundamentally understanding charge transfer process and theoretical prediction the charge mobility is important to design high efficient devices and will provide with helpful theoretical advices.Under the hopping framework, the aim of this dissertation is evaluating the charge mobility of organic molecular crystal by quantum chemistry method. The two key charge transfer parameters i.e. electron coupling and reorganization energy are calculated at different levels. We develop the isolated orbited method including dimer self-consistent field (SCF) and non-SCF method to calculate the charge transfer integral. Based on packing structure of molecular crystalline, the charge mobility is simulated by Einstein relationship method. The outline as following:In chapter one, we will briefly introduce the background of OLEDs including mechanism and some applications.In chapter two, we mainly introduce the basic electron transfer theory and the methods of modelling charge mobility in organic systems.In chapter three, benzene is chosen as benchmark to test the isolated orbital method based on the self-consistent filed (SCF). It is showed that isolated orbital method (SCF) can readily do the Lowdin's symmetric transformation to get reasonable coupling value. The basis set and distance dependent electron transfer integrals also are investigated by isolated orbital method. Compared with the accurate values from the searching minimum splitting between the two state based along the reation coordinates, the isolated orbital method with D95V basis set show very good performance in distance dependence.In chapter four, we evaluate charge transfer integrals of pentacene dimers for different space orientation under the isolated orbital method. The results indicate that the methods both SCF and non-SCF will give well charge transfer integrals in two kinds of different orientations. In particular, the SCF method not only gives effective charge transfer integrals but also can be used to investigate effect of site energy by molecular polarization. In addition, we also investigate the effect of electric filed on the electron coupling, the results show field has little effect on coupling and which can be used to calculate the rate of CT in the situation of no electric field.In chapter five, we choose the C1 symmertry with mer-Alq3 and C3 symmertry with fac-Alq3 as object. The charge mobility and electron spectra are investigated by quantum chemistry method. There are two parts here. We firstly based on the packing structure to simulate the charge mobilities for these two crystals. The results show that electronic carrier mobilities of crystal mer-Alq3 is more 100 times than hole carrier mobilities, which is a kind of good electronic transfer materials. However, the carrier mobilities of fac-Alq3 is just a little, because of the molecular orbital of mer-Alq3 mainly locate one ligand, but the molecular orbital of fac-Alq3 distribute to all ligands of molecular. In the second part, we simulate spectrum properties of different crystals, the results indicate mer-Alq3 give out of green light, but the spectrum of fac-Alq3 blue shift, which well agree with the experimental results.